1. Technical Field
The present invention relates generally to a roll mill used for wet-dispersion, and more particularly to a roll mill for use in milling-dispersing a substance, such as fine powder, or nano particles, in a material to be treated in production steps of various products such as ink, paint, ceramics, medicines, foods and electronic materials.
2. Background Technology
As an apparatus for a milling-dispersing treatment of a substance, such as fine powder or nano particles, in a material to be treated, plural rolls having different numbers of revolutions, for example, a triple roll mill having a front roll, a center roll and a rear roll arranged in parallel in a lateral direction, have been widely used. In such a roll mill, as described in, e.g., JP-UM-A-1-83438, a load exerted between the rolls is detected by a load sensor (load cell), and the front roll and rear roll are moved by a manual handle to adjust the distance between the rolls. If it is attempted to conduct an automatic control by, e.g., a servomotor instead of such a manual handle, the below-mentioned various problems are caused and therefore it is difficult to correctly conduct automatic control with only the load control by the load sensor.
As shown in explanatory FIG. 1, a roll mill for a milling-dispersing treatment of a material to be treated generally comprises a rear roll and a front roll movably mounted on a frame, and each roll has a roll shaft on each end portion thereof. Each roll shaft is rotatably mounted by a bearing (not shown), and a pressing force is applied via the roll shafts. Bearings for the roll shafts of a center roll, which is positioned between the rear roll and the front roll, are fixed on the frame. For ease of explanation, only the fixed center roll (lower roll) and movable rear roll (upper roll) are illustrated in FIG. 1, and the description of the rear roll applies as well to the front roll. Therefore, between the rolls, a pressing force “b” is exerted on a contact line of the roll on the pressing side and a reaction force “a” is generated on a contact line of the roll on the fixed side. Crowns R1, R2 are formed on the surfaces of the rolls so that the pressing force (contact reaction force) has a constant distribution (flat single line) on the contact line, not like a curve c or d as shown in FIG. 2. During operation of the roll mill, the pressing side roll and the fixed side roll are rotated at different speeds (i.e., the numbers of revolutions of the rolls are different from each other) so that a frictional force is generated between the rolls, and the frictional force plays a significant role for dispersion effects.
The pressing force (or reaction force) and the crowns on the roll surfaces are in a delicate relation with a certain mutual relation, and this mutual relation must be theoretically determined based on an accurate relation between cause and result. It has been found that the frictional force applied between both rolls influences the pressing force (or reaction force) and the influence cannot be disregarded as fluctuations of the pressing force. Namely, the relation is represented by: (extrapolated pressing force P1, P2)=(pressing force on a contact line)+(fluctuations caused by frictional force). This extrapolated pressing force is not itself the pressing force applied on the contact line.
Taking such a phenomenon into account, basically, a finite-element analysis model of a combination of two rolls being in contact with each other is prepared; a nonlinear analysis wherein a contact portion is progressively extended while the load is incrementally added on the contact line, is conducted; and based on the analysis, relations of the pressing force P1 from the rear roll, the pressing force P2 from the front roll, the crown R1 of the rear roll, the crown R2 of the center roll, the crown R3 of the front roll, a distance δ1 between the rear roll and center roll, and a distance δ2 between the center roll and front roll can be specifically obtained.
Further, regarding the frictional force between the rolls, it has been known that if a frictional force exists as mentioned above, it creates fluctuations and draws the required pressing force toward the incorrect side. Accordingly, the pressing force of each roll and the distance between the rolls is necessary to maintain the relations of P1, P2, δ1 and δ2 when no frictional force is applied, and it is apparent that the remaining parameters after removing P1, P2, i.e., δ1, δ2, should be used as indexes. As a result, the operation of the roll mill should be controlled by control of the displacement of the rolls in such a manner that the distance between the rolls δ1, δ2 determined at the initial stage of processing is maintained throughout operation of the roll mill.
More specifically, the roll mill is limited by a certain roll size and also the pressing force on dispersion processing required by the users. Using these parameters, at first, a static analysis of the roll is once carried out. From this result, it is possible to determine the configuration of the crown curve formed on the roll and the peak value of the curve (which usually exists at the center). Then, by incorporating the analysis results, conversion to a roll analysis model including the information of crown is carried out. Using this model, a contact analysis model wherein only the crown peak portions at the center are in contact with each other from the initial stage, is made. One of the rolls is a fixed roll, and its both ends are supported, and a constant load is applied from both ends of another roll, and in this manner, a finite-element nonlinear contact analysis is carried out in accordance with a load incremental analysis method. Accordingly, the load is finely classified into respective steps and finally reaches P1 or P2. The result shows the configuration represented by “constant reaction force” in the graph of FIG. 2. From this result, a distribution having a constant pressing force (or reaction force) on the contact line is obtained, and concomitantly R1, R2, R3, P1, P2, δ1, δ2 are obtained as interrelated numerical values. Among them, R1, R2, R3 are used as the crown amounts at the time of designating the rolls, and the rest, i.e., P1, P2, δ1, δ2 are numerical values used for automatic control.
As mentioned above, basically, the automatic control of a roll mill is preferably carried out by the control of displacement of the rolls by detecting the position of the rolls with a sensor. However, since the following problems are caused when using only control of displacement, it is necessary to conduct a partial correction by monitoring the load in addition to the control of displacement. At first, the following problem is caused by an unbalance of loads between the right and left ends of the roll. As shown in FIG. 3, usually, a contact line pressure between the rolls is that of a substantially flat distributed load at the portion that balances with the crown configuration formed on the roll. However, in actuality, in FIG. 3, when A, B are fulcrums, C is a center point, AC=L2 and CB=L1, and the distributed load is a little larger at the center point C where AB/2=L1=L2.
Further, since it is better for simplification of analysis to assume that the distributed load is replaced with a concentrated load at the center point C, explanation will be made hereinafter on an assumption that a concentrated load P2 (P1) is applied at this point. Then, it is clearly understood that moments P2×L1 and P2×L2 are applied at both sides A,B of the point C. However, the distribution of contact line pressure may sometimes be shifted as shown in the lowest part of FIG. 3 due to a disturbance occurring during operation of the roll mill. At this time, the position of P2 may be shifted from the center point C, for example, L1>L2 (of course, L2>L1 may happen). If load control works at this time, the total P2 does not change, and as a result, the above moments become P2×L1>P2×L2 and a moment difference between the left and right sides is caused. In this instance, due to the moment difference, P2 is pulled toward the L1 side (B side), i.e., a larger moment side. If the relation is contrary, P2 is pulled toward the L2 side (A side). Accordingly, a force always works so that P2 remains at the center point C, and P2 acts as a so-called self-alignment force.
However, if the control is made by a feedback control only with the control of displacement, the above-mentioned self-alignment force does not work in this mechanism. Specifically, although an unbalance of load between the left and right ends of the roll can be corrected by the control of displacement, some phenomenon has been often seen in actual operation with the control of displacement only, wherein when a minute unbalance is caused between both ends of the roll, although no substantial error in numerical values of displacement is recognized, a minute unbalance in terms of the load is often detected.
The relation between load and displacement under actual operation and the relation between load and displacement under static load will be considered below. When the pressing force P2 (P1) is applied to the roll as mentioned above, the roll contacting parts momentarily deform and have collapsed portions as shown in the cross-sectional view of FIG. 4. Namely, when the radius of the roll is R and the distance between the rotational centers of the roll shafts is D in FIG. 4, 2R>D. When the collapse allowance at this time is 2R−D=2d, “d” is the collapse allowance of one roll. The control is made by employing D as the distance between rolls δ1 (δ2).
At the time when raw materials are fed in the machine, actual operation for control of displacement is carried out by employing “D+e” as the actual numerical value for control of displacement taking the clearance “e” where the raw materials are nipped into account, not by employing the distance D between roll shafts. Accordingly, it is required to see whether or not the load agrees with a statistically determined P2 (P1) under actual operation with the control of displacement, and it is also required to detect a monitor value of a load cell and to operate with D+e that agrees with P2 (P1). For this purpose, in such instance, the control is carried out by employing D+e as the distance between rolls δ1 (δ2).
Generally, “e” is called a nip in the technical field of roll mills, and more specifically, the nip of the material-feeding side (first clearance) is called a feed nip, and the nip of the material-dispersing side (secondary clearance) is called an apron nip. Both the feed nip and the apron nip are referred to as simply “nip” hereinafter.
Further, since materials to be treated are dragged into the nip between rolls during a milling and dispersing operation, a film thickness of the materials through the nip at the initial stage is “e” which is the same as the clearance “e”. However, it is known that the film thickness e reduces as the milling and dispersing operation proceeds. When the change of the film thickness e with time is empirically expressed as a function with respect to time e(t), the distance between rolls δ1 (δ2) can be controlled as D+e(t).
Furthermore, it is a well-known fact that when the materials to be treated are fed in a triple roll mill, depending on the properties of the materials, the viscosity of the materials tends to decrease as the dispersion proceeds. Assuming that this machine is used by cycling the materials in a multiple number of passes, if only control of displacement is used while the viscosity decreases, the load applied to the materials may possibly decrease as the number of passes increases, resulting in ineffective operation. At present, triple roll mills are typically used in multiple-number-of-pass operations. In such cases, it is required to utilize a load cell (load sensor) monitor, and if the mechanism of the machine is designed so that when the reduction of the viscosity of materials falls below a prescribed value, correction can be made with a program without external processing, whereby users can conduct the desired dispersion by a one-step dispersion treatment. As explained above, although the operation control is made by the control of displacement, a mechanism is provided wherein the load is monitored by a load cell (load sensor) and fluctuations are adjusted by a program.
Furthermore, there is a problem when an abnormal load occurs. In operation of a triple roll mill, if a substance larger than normal (i.e., larger than that for which the parameters of the rolls have been designed) is erroneously inserted between the rolls, a control mechanism using only control of displacement will cause a vast load to be applied so as to keep the roll displacement within preselected limits, which may disrupt the operation of the machine or damage the machine. In order to deal with such a problem, a load cell (load sensor) for measuring a pressing force is inserted into a control system, by which a program can be constructed wherein when a trigger-type abrupt unavoidable displacement happens, application of a vast load can be avoided by a rapid feedback so as to protect the machine.
The load cell is effective for incorporating so-called safety measures for avoiding abnormal load. In addition to the above case where a substance larger than normal is erroneously inserted between the rolls, a substantial difference in displacement may be seen by a uniform distinctive variation of viscosity or uneven distribution of materials to be treated on the rolls. It has also been confirmed that when such things continuously happen, the control of displacement is a little inferior to the control of load in view of the temporal efficiencies in control for constricting this difference.
Accordingly, in order to avoid such redundancy and obtain rapid control, it is more preferred that the program be constructed so that when a large, but not a level of emergency shutdown, disturbance is caused, the control is temporarily changed from the control of displacement to the control of load, and immediately after constraint of the disturbance, the control is returned from the control of load to the control of displacement.